A feature called Intra-domain connection of Radio Access Network nodes to multiple Core Network Nodes was included in release 5 of 3GPP standards. The specification 23.236 {3GPP TS 23.236 3rd Generation Partnership Project Technical Specification Group Services and System Aspects: Intra-domain connection of Radio Access Network (RAN) nodes to multiple Core Network (CN) nodes} defines how radio access network nodes (Base Station Controllers (BSCs) or Radio Network Controllers (RNCs)) can connect to more than one core network node (Mobile Switching Centre (MSC) or Serving GPRS Support Node (SGSN)) at the same time instead of just one at a time as was previously the case when each BSC or RNC was connected to one and only one SGSN or MSC.
A radio access network node chooses a core network node to handle a particular user communications device such as a Mobile Station (MS). This procedure is called Network Access Server (NAS) Node Selection which for the case of packet switched part of the network equates to SGSN Selection. The procedure aims to distribute MSs between the SGSN nodes in a fair manner.
After a SGSN has been chosen for a particular MS in a Global System for Mobile (GSM) network, the SGSN allocates a Packet-Temporary Mobile Subscriber Identity (P-TMSI) to the MS. The P-TMSI is contained within a Temporary Logical Link Identifier (TLLI). A part of the P-TMSI contains the Network Resource Identifier (NRI), which identifies a SGSN. The NRI is then used by the BSC to route uplink packets to the correct SGSN.
It may arise that an MS may not have a P-TMSI. It may be the first time the MS attaches or the MS may have for some reason lost the P-TMSI). In the allocation procedure the MS is mandated to send a random number instead of the P-TMSI, a so-called random TLLI. This random number is used by the MS until the SGSN has allocated a P-TMSI. Each time a BSC encounters a random TLLI (or the received NRI does not correspond to a reachable SGSN) the BSC performs a deterministic function in order to choose a SGSN to be responsible for the subsequent handling of this MS.
In Packet Switch GSM systems it has been noted that some brands and makes of MSs are not adhering to the standard. Instead of using a proper random TLLI value when attaching, the MSs are using a fixed value TLLI. The effect this has is that the MSs that use the same “static random TLLI” are all concentrated to one and the same SGSN in an SGSN pool by SGSN selection procedures in the BSC. The selection procedures rely on the MSs to adhere to the standard of using a random TLLI when they first attach to the network. In turn this leads to a severely skewed distribution of load between the SGSNs in the SGSN pool, causing some SGSNs to face an overload situation whereas others remains underutilized. One or a few SGSNs can be brought to the verge of overload whereas others only exhibit as moderate or low load.